|1.||Tractor for the towing of aircraft and of the type which supports and holds the nose wheel of the aircraft within the supporting surface of the tractor, and in which the steerable wheels of the tractor are provided with po¬ sitive caster, c h a r a c t e r i s e d in means for totally or partly putting the normal steering system of the tractor out of operation by towing at speeds above a certain limit, and in that the steerable wheels are allowed to swing freely, possibly within certain limits.|
|2.||Tractor according to claim 1, c h a r a c t e r i s e d in that the front wheels, at speeds above a certain limit, have a steering function only, and that the rear wheels under these circumstances have a driving and braking function only.|
|3.||Tractor according to claims 1 or 2, and provided with power steering for the steerable wheels, c h a r a c t e ¬ r i s e in means for putting the power steering out of operation when the speed exceeds a certain limit.|
|4.||Tractor according to claims 1 or 2, and provided with power steering for the steerable wheels, c h a r a c t e r i s e d in means for graduating the power steering in such a way that it will decrease gradually with increasing speed.|
|5.||Tractor according to claims 3 or 4, c h a r a c t e ¬ r i s e d in that the steering wheel is shaped as a closed, smooth circular disk, possibly with an annular bead along the rim.|
|6.||Tractor according to one of more of the preceding claims, c h a r a c t e r i s e d in a play of for example 5° in the steering system, said play being springloaded towards the middle position.|
|7.||Tractor according to claim 6, c h a r a c t e r i s e d in that a shock absorber is furthermore built in, in connection with the spring or springs.|
|8.||Tractor according to claim 7, c h a r a c t e r i s e d in that the shock absorber is adapted to lock in a certain position, preferably at speeds below the said limit.|
|9.||Tractor according to one or more of the preceding claims, c h a r a c t e r i s e d in means for con¬ tinuously sensing the longitudinal force between the tractor and the nose wheel, and in means, on the basis of the sensed signals, for keeping the transferred force within an acceptable interval by appropriate adjustment of the effect of the tractor, including the braking effect.|
The invention relates to a tractor for towing aircraft and of the type, which can support and secure the nose wheel of the aircraft within the supporting surface of the tractor, and in which the steerable wheel of the tractor is provided with positive caster.
When towing large traffic aircraft by means of a tractor with the nose wheel of the aircraft supported by the trac¬ tor there is a risk of overloading the undercarriage of the nose wheel by horizontal forces, which can for in¬ stance occur if the pilot performs a braking operation by means of the other landing wheels of the aircraft.
The specification to US Patent no. 4.113.041 and SE Patent application no. 8.604.055-7 concern techniques for solving the problems of the longitudinal horizontal forces, which can affect the nose wheel. The sideways forces can occur as a consequence of uneven braking of the starboard and port main wheels respectively. The specification to EP Pa¬ tent application no. 84.903.171.1-2306 describes a method and a tractor, whereby the problems of the sideways or transverse and longitudinal, horizontal forces can be solved. However it has proven to be difficult with the techniques indicated to secure the nose wheel to the tractor and at the same time measure the transverse forces occurring.
The purpose of the present invention is to provide a further development of the techniques, the object of which is to keep the said transverse forces within acceptable limits.
The tractor according to the invention is characterized in means for totally or partly putting the normal steering
system of the tractor out of operation at speeds above a certain limit, viz. the steerable wheels are allowed to swing freely, possibly within certain limits.
The tractor will hereby be forced to follow the movements of the nose wheel transversely to the direction of driving, as the tractor will move in the same direction as the resultant of the horizontal forces which are transferred through the nose wheel. In practice this means that the pilots of the aircraft will decide the course of the tractor in just the same way as when the aircraft is driven by itself without a tractor at high speed, that is, by applying the rudder and the brakes of the main wheels, but without applying the steerability of the nose wheel. This construction is considerably simpler than the known construction, since it is unnecessary to secure the nose wheel by a complicated mechanism to make it possible to measure the transverse forces.
The tractor may furthermore be characterized in that the front wheels, at speeds above a certain limit, have a steering function only, and the rear wheels have a driving function only.
As is well known, it is unfortunate to steer and to trans¬ mit an effect through the same wheels at the same time, and in consideration of the stability it is therefore ad¬ vantageous that the front wheels are not driving, es¬ pecially because a high degree of positive caster of the front wheels will be chosen.
If the steerable wheels of the tractor are provided with power steering, said tractor may be characterized in means for putting the power steering out of operation, when the speed exceeds a certain limit. A very simple way of realizing the invention, and thus very inexpensive, is
According to the invention the control wheel of the trac¬ tor may be shaped as a closed, smooth, circular disk, pos- sibly with an annular bead along the rim. In this way in¬ jury to the tractor driver can be avoided in case forced steering occurs as a result of strong transverse forces on the nose wheel.
Another embodiment of the invention, which, however, does not exclude the above mentioned, consists in that a play of for example 5° or a damping device being spring- loaded towards the middle position, is built into the steering system.
Furthermore, a shock absorber may be built in in connection with the spring or springs, according to the invention.
The shock absorber according to the invention can be adap¬ ted for locking in a set position, preferably at speeds below the said limit.
Finally the tractor may be characterized in means for con- tinuous sensing of the longitudinal forces between tractor and nose wheel, together with means, based on the sensed signals, for keeping the transmitted force within a per¬ mitted interval by appropriate control of the effect given off by the tractor, including the braking effect. The undercarriage of the nose wheel is hereby completely safeguarded against unacceptable overloads, while the pi¬ lots can still at any time manoeuvre the aircraft in the same way as if it were driven freely and with the same means of manoeuvering.
The invention is explained more detailed with reference to
the drawing, which shows schematically a top view of an embodiment of the tractor according to the invention.
The tractor shown has four wheels 1, 2, 3 and 4, two of which 1 and 2 are steerable front wheels, the appro¬ priate steering knuckle arms 3a and 4a being connected to a steering connection rod 5. The wheel 1 is connected to a steering arm 6 through a steering rod 7 in which is inser¬ ted a piston cylinder 8, the steering rod 7 being connected to the housing of the cylinder 8, while the piston 9 is connected to the steering arm 6 through a piston rod 10. The piston 9 is in its idle state kept in the shown middle position by compression springs 11 and 12.
The piston cylinder 8 is filled with shock absorber liquid, and the piston 9 is provided with a hole 13, which can be opened and closed by a magnetic valve (not shown) controlled by loose wires 14 and 15, which are activated by a speedometer in such a way that the hole will be opened at speeds above a certain limit, for example 30 km/h. The arrows 16 and 17 symbolize a steering moment, which makes the steering arm 6 swing in a per se known way.
The steerable wheels 1 and 2 are provided with positive caster, which means that the swivel axis of each wheel in¬ tersects the road surface in front of the point of contact of the wheel. As it is generally known, the wheel will he- reby have a self-correcting tendency, which will make it roll in the direction of movement.
The tractor works in the following way.
During slow towing in the vicinity of the airport buildings and other aircraft the piston will be locked in
the position shown, and the tractor driver will be able to manoeuvre the tractor in the usual way.
By faster towing, for example at speeds above 3o km/h and during straight driving on the runway, the piston is not blocked. The pilots of the aircraft will now be able to correct the course by just braking gently with one set of wheel brakes, for example starboard wheel brakes. This will result in a transverse force on the tractor trans- mitted through the nose wheel which is secured to the top of the tractor. This transverse force will cause the front wheels 1 and 2 to veer a few degrees to the right as a re¬ sult of the positive caster of said wheels. This will immediately result in a compression of the spring 11 and of course in a change of course of the tractor, which the driver will not be able to prevent in the first round.
When the tractor then yields to a transverse force on the nose wheel it is possible to reduce the maximum transverse force possible and hereby protect the leg of the nose wheel from overloading.
Whether the tractor driver, upon observing the change of course, will be able to bring the tractor back to the old course, will naturally depend on the steering forces which he can impart on the system. The essence of the present invention is that in this situation the tractor driver should not be able to influence the steering. Naturally this can be be implemented by a suitable dimensioning, for example by chosing an appropriate exchange or by putting the power steering system out of operation at speeds above a certain limit. Obviously, there should be no self-locking cog wheel connections or worm gears in the steering system.
Tractors of the above mentioned kind are usually steerable
on all wheels, and it is possible to choose between three steering configurations. In the first place, the one shown with steerable front wheels. In the second place, the so called crab steering in which all wheels are coupled in synchronous movement to steer in the same direction. Fi¬ nally, a configuration in which the rear wheels steer the opposite way as compared to the front wheels.
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